1. Field of the Invention
The present invention relates to a stamper using an inorganic resist, and more particularly, to a light transmissive stamper.
2. Description of the Related Art
In recent years, an optical recording medium is employed as a recording medium for recording various informations in, for example, the audio visual field including computers. Also, along with the populization of mobile computers and diversification of information, there is a need for a small optical recording medium of a large capacity.
The optical recording medium is formed with a recording film on a substrate and recording is performed on the recording film through an optical energy, which provides a signal pattern which is read by a drive. For protection of the film surface, the recording film is made through bonding of an organic protective layer and a substrate to constitute a optical recording medium. Moreover, in order to increase the capacity, there is employed, for example, an optical recording medium in which substrates each having a recording film are opposed to each other and are bonded.
However, a still larger capacity is required for an optical recording medium for high-definition contents in terrestrial digital broadcasting, or the like. The increase of track recording density of an optical disk greatly depends on a laser wavelength λ of a reproducing optical system and a numerical aperture NA of an objective lens. The detectable limit of a spatial frequency at the time of reproducing a signal is approximately NA/λ. Accordingly, in order to realize the increase of the density with the existing optical disks, the laser wavelength λ of the reproducing optical system can be shortened and the numerical aperture NA of the objective lens can be increased, thereby increasing the track recording density. Setting an aperture ratio larger enables reduction in a beam spot size to the wavelength limit. However, the stability against the inclination of the optical recording medium is remarkably reduced, so that the thickness of the substrate needs to be reduced. Such a technology is adopted so that the substrate on the light incident side is made thin as an optical recording medium, and a recording medium of a large capacity is commercialized (for example, Blu-ray Disk (trade name; hereinafter referred to as “BD”)).
Also, the fine processing technology for improving the storage capacity of an optical disk is dramatically advanced through a mastering technology of performing a pattern formation of the optical disks. The advance has been achieved by shortening an exposure wavelength of an exposure device used in mastering, improving accuracy in exposure pattern control, and improving sensitivity of a photoresist.
However, in a production method in which an existing photoresist is used, it takes four hours or more to expose a single master stamper, which is a precision mold and used in BD production. Accordingly, studies have been conducted to establish a high speed production process.
In order to solve the problem, a method which does not use an existing resist formed of an organic material but uses a resist formed of an inorganic material has been proposed and studied in various companies.
For example, Japanese Patent Application Laid-Open Nos. 2003-315988 and 2004-152465 propose a master disc for a stamper using an inorganic material composed of an incomplete oxide of a transition metal as a resist. In these patent documents, it is found that when the oxygen content in a transition metal oxide is made less than the oxygen content of a stoichiometric composition corresponding to valence which the transition metal can take, absorption of an ultraviolet ray or a visible light of the oxide (incomplete oxide) is increased. According to the inventions in the above-mentioned patent documents, by using an incomplete oxide of the transition metal as a resist material, fine exposure and development applicable to the production of the BD can be conducted, using an ultraviolet ray or a visible light as an exposure source. Further, exposure at the rate of 4.92 m/sec, which is equal to the rate of recording/reproduction in the BD, is enabled and the exposure rate can also be improved dramatically.
Further, tungsten oxide (WO) and molybdenum oxide (MoO) used as an inorganic resist has transmissivity with respect to an ultraviolet ray used when curing a photocurable resin and can be therefore used for formation of an intermediate layer (photocurable resin is mainly used) of an optical recording medium having a multilayer structure. For example, by using a material having transmissivity such as quartz glass as a support substrate of a stamper, and by providing, on the light transmissive support substrate, a light transmissive inorganic resist layer having an uneven pattern formed thereon, ultraviolet ray irradiation from the stamper side is enabled when an intermediate layer of an optical recording medium having a multilayer structure is formed, which is advantageous.
Moreover, in general, the stamper using an inorganic resist exhibits an excellent flatness such that an electrocasting process or polishing process is not necessary. Also from this viewpoint, the stamper using an inorganic resist is particularly advantageous for forming an intermediate layer of an optical recording medium having a multilayer structure (hereinafter, abbreviated as “intermediate layer”), which is required to have flatness.
A manufacturing process of a stamper using an inorganic resist is described below. First, a support substrate for a stamper is cleaned. Glass or a Si wafer is employed as the support substrate. Next, an inorganic resist layer is formed on the support substrate by sputtering to produce a master disk of a stamper. Subsequently, exposure of a predetermined uneven pattern is performed with a laser beam on the thus produced master disk of a stamper, and then alkaline development is performed to produce a stamper having an inorganic resist layer which has an uneven pattern formed thereon, on the support substrate.
However, an inorganic resist is not formed in a side surface portion of the stamper produced by the above-mentioned procedure, and the support substrate is exposed in the side surface portion of the stamper. When the stamper having the support substrate exposed in the side surface portion thereof is used to form a substrate or an intermediate layer by using a curable resin as a material through a photopolymer method (hereinafter, abbreviated as “2P method”), the curable resin which flows out to the side surface portion of the stamper at the time of molding and the support substrate are adhered to each other, which causes difficulty in peeling the substrate or the intermediate layer from the stamper. Besides, the curable resin is adhered to the exposed support substrate after peeling and remains on the exposed support substrate, so that a residue of the curable resin are generated on the side surface portion of the stamper. In the case where a residue of the curable resin is generated on the side surface portion of the stamper, a localized gap is caused between the stamper and the substrate or the intermediate layer at the time of molding. As a result, the curable resin cannot be spread uniformly over the entire stamper surface and satisfactory molding cannot be performed. Incidentally, a resin which cures with an ultraviolet ray or a visible light, such as an acrylic resin, a methacrylic resin, or an epoxy resin, is generally employed as the curable resin.
For that reason, when a residue of a curable resin is generated on a side surface portion of a stamper, a step of removing the residue and a step of cleaning the stamper are indispensable, which remarkably degrades the productivity. In addition, there is a problem that the lifetime of the stamer is shortened and the production cost increases.
In particular, in the case where glass is used as the support substrate and an ultraviolet curable epoxy resin is used as the curable resin, occurrence of the residues is remarkably apparent. It is considered that this is because the glass (support substrate) which is exposed in the side surface portion of the stamper and the curable resin which flows out at the time of molding a substrate are reacted with each other to cause a strong adhesion force.
Further, in the case where a molded substrate and the stamper has a substantially equal diameter, or the stamper has a smaller diameter than the molded substrate, it is difficult to completely prevent flowing out of the curable resin to the stamper side surface at the molding of the substrate, so that molding defects or residues are liable to occur.
That is, in the conventional stamper using an inorganic resist, the exposure of the support substrate in the side surface portion of a stamper is not taken into account. Therefore, in the stamper using an inorganic resist, there has been required a method in which a residue of a curable resin is not generated in the stamper side surface at the molding.
As described above, in the stamper using an inorganic resist, the support substrate is exposed in the side surface portion thereof. Therefore, there is posed a problem that in the case where such a stamper is used to form a substrate or an intermediate layer through the 2P method, the support substrate which is exposed in the stamper side surface and the curable resin which flows out at the molding are adhered to each other, which causes a residue in the side surface portion of the stamper.